This feature is likely to require an RFC. If you're interested in
having it, I suggest you read and follow
<https://wiki.php.net/rfc/howto>. :)

I am not sure why you mention French. France like all Europe, US, South America and other countries use the same calendars.

I cannot remember why we introduced this function (way back), but as of now, I do not see a point to have them as it fits perfectly in a custom library or extension, using INTL (ICU), which supports it:

http://icu-project.org/apiref/icu4j/com/ibm/icu/util/EthiopicCalendar.html

You can still propose it via a RFC tho'

Thank you for your interest in PHP and for submitting a feature
request. Please be aware that due to the magnitude of change this
request requires, it would be necessary to discuss it on PHP
Internals list (internals@lists.php.net) as an RFC. Please read
the guide about creating RFCs here:
<https://wiki.php.net/rfc/howto>. If you haven't had experience
with writing RFCs before, it is advised to seek guidance on the
Internals list (<http://php.net/mailing-lists.php>) and/or solicit
help from one of the experienced developers. 

Please do not consider this comment as a negative view on the
merits of your proposal – every proposal which requires changes of
certain magnitude, even the very successful and widely supported
ones, must be done through the RFC process. This helps make the
process predictable, transparent and accessible to all developers.